1. Field of the Invention
The present invention relates to back light modules, and more particularly, to back light modules whose outer portion has higher luminance than inner portion thereof.
2. Description of the Prior Art
A back light module is one of the key components for the liquid crystal display (LCD) or the scanner. The back light module comprises light sources and other optical devices for reflecting or refracting light to provide uniform light output. The light source of the back light module is typically a cold cathode fluorescent lamp (CCFL) or light emitting diode (LED). In general, the back light module can be divided into two groups, the edge lighting type and the direct lighting type. The two groups are categorized by the positions of the light sources thereof.
In the conventional art, the main concept of designing both the edge lighting type and the direct lighting type is to provide a uniform light output from the back light module, i.e., to uniform the luminance distribution of the back light module. Unfortunately, such a design concept may negatively affect the performance of some applications.
For example, suppose that a conventional back light module having uniform luminance distribution is employed in a scanner as a back light source required for scanning transparencies, and positive or negative films. The brightness of the outer portion of a scanned image received by an optical module of the scanner is usually lower than the brightness of the inner portion of the scanned image due to the optical characteristics or mechanical designs of the optical module, and thereby reducing the scanning quality of the scanner.